1. Field of the Invention
The field of the invention is that of overhead conveyers having fixed axis, rotatable drive shafts engaging skewed driven wheels of a carriage to provide the carriage conveying force, wherein the carriage is supported by the drive shaft and/or a fixed support rail.
2. Description of the Related Art
The present invention relates to overhead conveyors of the type disclosed in U.S. Pat. No. 5,806,655 issued Sep. 15, 1998 to Tabler, in U.S. Pat. No. 5,785,168 issued Jul. 28, 1998 to Beall, Jr., in U.S. Pat. No. 4,203,511 issued May 20, 1980 to Uhing, in U.S. Pat. No. 3,164,104 issued Jan. 5, 1965 to Hunt, and in U.S. Pat. No. 3,850,280 issued Nov. 26, 1974 to Ohrnell.
Conventional rotating shaft driven overhead conveyors are limited in the amount of weight they may carry or the incline/decline they may traverse, without the carriage undergoing an uncontrolled slippage, particularly between the drive shaft and the driven wheels, Therefore, overhead conveyors for loads or inclines too great for the rotating shaft driven overhead conveyer are generally of a different type, for example a power and free chain driven conveyor.
Shaft driven overhead conveyors have many advantages over the heavier load type conveyors such as the power and free conveyor; such advantages including quietness, cleanliness, less repair, easy diversion of load carrying carriages, buffering, speed variation along the conveying path, and generally greater flexibility in design.
This well known slippage problem of the rotatable drive shaft type of overhead conveyor has been partially solved by sand-blasting and then anodizing aluminum drive shafts, which adds expensive processing to the manufacturing. Though this is an improvement for some applications, in many cases, it is not enough.
In addition to ascents and to a lesser extent, descents, the problem arises in other circumstances, for example: when a carriage with spaced apart trolleys for a single load (two trolleys being used to carry a greater load than can be carried with a single trolley) passes through a switch. Where a carriage traveling straight on one line, is switched to travel on another line, a trolley passing through the switch may not be powered, so that the rear trolley is the sole drive into the switch and the front trolley is the sole drive out of the switch. In such a situation, the driving power is cut in half through the switch and slippage is more likely to occur, for example when the load is particularly heavy in the high load overhead conveyor of U.S. Pat. No. 5,785,168 issued Jul. 28, 1998, whose disclosure is incorporated herein in its entirety, by reference.
Therefore, there is a need to increase drive friction between the drive shaft and driven rollers of the rotatable drive shaft type of overhead conveyor systems at least at selected locations of a system where a slippage problem is most likely to occur.